3rd Grade · Measurement & Data
3.MD.D.8: Find the perimeter, the distance around a shape, or a missing side
3.MD.D.8 is a 3rd grade Common Core math standard in Measurement & Data. Here’s what it really covers, and how you can help your students learn it.
By Robert “Bobby” BarnettMath teacher, parent, and founder of Bobby’s Math Club
The standard
Solve real world and mathematical problems involving perimeters of polygons, including finding the perimeter given the side lengths, finding an unknown side length, and exhibiting rectangles with the same perimeter and different areas or with the same area and different perimeters.
In plain words
Find the perimeter, the distance around a shape, or a missing side.
Sample questions from student practice
These questions from Bobby's Math Club are the kinds of questions your students will see. You can try answering them to see what a student might see.
Question 1 of 3
A rectangular garden has sides that are 5 feet long and 3 feet wide. What is the perimeter of the garden?
- 8 feet
- 15 feet
- 16 feet
Where students can get stuck
- Adding only the two side lengths shown, so a 5 by 3 rectangle gets a perimeter of 8 instead of 16.
- Multiplying the sides to find perimeter, mixing it up with area, so a 5 by 3 rectangle gets 15.
- Assuming equal perimeters mean equal areas, so a 7 by 1 rectangle and a 4 by 4 square seem the same size.
Effective teaching strategies
- Have students trace a shape with a finger and mark each side as they add, so every side is counted, including the sides that are not labeled.
- Give students a triangle with a perimeter of 12 and two sides of 3 and 4, and have them find the missing side, then add all three sides to check.
- Challenge students to draw two rectangles with the same perimeter but different areas, such as 7 by 1 and 4 by 4.
Units that teach this standard
Find where 3.MD.D.8 lands in the program you teach. Each unit has a guide written for families. Unit names and order vary between editions and districts, so your school’s own materials have the exact sequence.
Bridges in Mathematics
enVision Mathematics
i-Ready Classroom Mathematics
Illustrative Mathematics
Math Expressions
Reveal Math
Don’t see your curriculum? Contact Bobby and we’ll see if we can help!
Share it with families
Families can read Perimeter: the distance around a shape for a plain-language look at this skill, with three questions to try together and ideas for helping at home.
Help every student practice measurement and data at home
In Bobby's Math Club, students practice at just the right level, with a hint when they need one, a clear explanation after every question, and kid-friendly videos. Unlimited practice is free for every family.
See how it works for teachers
Written by Robert “Bobby” Barnett
Bobby is an award-winning math teacher, the author of Meet Every Learner’s Needs, and the father of two elementary-school math learners. He uses Bobby’s Math Club with his own kids a few times every week. Learn more or contact Bobby.